1. Field of the Invention
The present invention relates to a combustion-powered, fastener-driving tool for driving fasteners, such as nails, rivets, or staples. The combustion-powered, fastener-driving tool includes a cylinder in the top section of which is formed a combustion chamber. The combustion-powered, fastener-driving tool generates a motive force for driving a piston in the cylinder by igniting a mixture of air and a flammable gas in the combustion chamber.
2. Description of the Related Art
Conventional combustion-powered, fastener-driving tools is disclosed, for example, in U.S. Pat. Nos. 5,197,646 and 4,522,162. Such conventional combustion-powered, fastener-driving tools typically include a housing serving as the main enclosure of the tool, a cylinder accommodated in the housing, a piston disposed in the cylinder and guided by the cylinder to move vertically in a reciprocating motion, a driving blade fixed to the piston for driving a fastener into a workpiece when the piston moves in a downward operation, a combustion chamber frame provided in the housing that slides vertically while guided by the periphery of the cylinder, the combustion chamber frame forming a combustion chamber having walls defined by the combustion chamber frame and the piston when the combustion chamber frame is moved upward, an injection opening for injecting a flammable gas from a gas cylinder accommodated in a grasping portion or a handle into the combustion chamber, a fan provided in the combustion chamber, a spark plug for igniting a mixture of air and the flammable gas injected into the combustion chamber, a trigger mounted on the handle, and an ignition system electrically connected to the trigger for producing a spark in the spark plug when the trigger is operated.
The combustion-powered, fastener-driving tool having this construction supplies a mixture of the flammable gas from the gas cylinder mounted on the housing and air to the combustion chamber. The combustion-powered, fastener-driving tool generates a spark with the spark plug in the combustion chamber when the trigger is operated to detonate the mixture in the combustion chamber. The resulting explosion generates a driving force for driving a nail or other fastener. Unlike a compressed-air, fastener-driving tool that uses compressed air as a driving source, this combustion-powered, fastener-driving tool requires no compressor and is, therefore, much easier to transport to a construction site or the like. Further, the combustion-powered, fastener-driving tool can be conveniently provided with an internal power source, such as a battery, so that the tool can be used in any environment without requiring a commercial power supply.
FIG. 10 shows the ignition ratio with respect to the concentration of fuel in the combustion chamber (ratio of flammable gas to the total volume of the combustion chamber) for the conventional combustion-powered, fastener-driving tool. The result shown in FIG. 10 is obtained at a circumstance where the external temperature of the tool is maintained at constant, such as 25° C., and when the same type of fuel is used and the spark intensity of the spark plug is maintained at constant. Inventors have found that the ignition ratio varies depending primarily on the gas type, temperature, and spark intensity.
As shown in FIG. 10, the ignition ratio is 100% when the gas concentration is within a specific range (hereinafter, this range will be referred to as a “gas concentration band”). The gas concentration band in the example of FIG. 10 is the range of 3.4-6.5%. The mixture of liquefied gas and air in the combustion chamber in this gas concentration band ignites reliably.
However, a spark from the spark plug cannot reliably ignite a gas concentration outside of the gas concentration band, that is, when the ignition ratio is less than 100%. In fact, the gas in the combustion chamber does not ignite at all when the concentration of gas separates farther from the upper or lower limits of the gas concentration band. Hence, there is a demand to expand this gas concentration band at which the ignition ratio is 100% in order to ensure stable ignition.
However, the amount of liquid injected from the gas cylinder is easily influenced by temperature inside the faster-driving tool or external air temperature. Such changes in the amount of liquid gas injected at a low temperature or a high temperature may result in a gas concentration outside of the gas concentration band, making it impossible to ignite the mixture reliably. This unreliable ignition is likely due primarily to a flameout phenomenon in which the electrode of the spark plug robs the heat from the spark.